Extensive networks of underground electrical cables are in place in many parts of the industrialized world. Such underground distribution offers great advantage over conventional overhead lines in that it is not subject to wind, ice or lightning damage and is thus viewed as a reliable means for delivering electrical power without obstructing the surrounding landscape, the latter feature being particularly appreciated in suburban and urban settings. Unfortunately, these cables (which generally comprise a stranded conductor surrounded by a semi-conducting conductor shield, a polymeric insulation jacket, and an insulation shield), particularly those installed prior to 1985, often suffer premature breakdown and do not attain their originally anticipated longevity of 30 to 40 years. Their dielectric breakdown is generally attributed to so-called “treeing” phenomena (i.e., formation of microscopic voids or branching channels within the insulation material, from which the descriptive terminology derives), which lead to a progressive degradation of the cable's insulation. Since replacing a failed section of underground cable can be a very expensive and involved procedure, there is a strong motivation on the part of the electrical utility industry to extend the useful life of existing underground cables in a cost-effective manner.
A typical method for rejuvenating in-service cables comprises introducing a tree retardant fluid into the void space (interstitial void volume) associated with the strand conductor geometry. This fluid, which diffuses into the insulation and fills the microscopic trees to augment the service life of the cable, is generally selected from a particular class of aromatic alkoxysilanes which can polymerize within the cable's interstitial void volume, as well as within the water tree voids in the insulation (Vincent et al. in U.S. Pat. No. 4,766,011). This method and variations thereof employing certain rapidly diffusing components (U.S. Pat. Nos. 5,372,840 and 5,372,841) have enjoyed commercial success over the last decade or so. However, all of the current methods known to applicants still do not deliver the full potential of insulation longevity. This is very likely due to the diffusion of most of the currently used fluids out of the cable within 10 to 15 years after treatment, thereby again exposing the cable to the above mentioned treeing phenomena (e.g., see Bertini, “Accelerated Aging of Rejuvenated Cables—Part I”, ICC, Sub. A, Apr. 19, 2005). Thus, there is a continued desire on the part of the utility industry to further extend the reliable performance of treated cable, thereby improving efficiency and reducing operating costs.